split policies into separate .rs files

[mcandre]

For ease of maintenance.

And replace the terms "policy" / "pplicies" with "check" / "checks". (We would have used "rule" / "rules", but that collides with make rules and PEG grammar rules.

[mcandre]

Unknown if this is possible in Rust. There seems to be an assumption that each *.rs file declares at most one module, unlike Go.

Meanwhile, what we can do is move the unit tests closer to the entities that they test.

[mcandre]

The least boilerplate way of accomplishing the file splitting:

1. Isolate the Warning struct and the mock utility function in a new module. Isolate the lint function in a lint.rs file.
2. Start isolating each of the checks in a new checks/<check>.rs file.
3. Re-import each of the individual checks in the lint module.